Venezuela's president Hugo Chávez sits beside his vice president Nicolas Maduro as he makes a televised address announcing he will undergo further cancer treatment Photograph: Marcelo Garcia/AP

Venezuelan president Hugo Chávez announced on Saturday night he will return to Havana to undergo surgery for the fourth time following the discovery of new cancer cells during his trip to Cuba last week.

During a 30 minute nation-wide address, the socialist leader said that vice president Nicolas Maduro is to step in his place while he remains away.

"I need to, I must return to Havana tomorrow", he said. "In order toconfront this new battle...

"I have complete faith that with God's help we will emerge victorious. We have always lived from one miracle to another," he added.

Chávez, who was recently re-elected for a fourth period, also said that if he were unable to assume power in the near future Venezuelans should vote for Maduro in the elections that according to the Constitution must be called in the 30 days following a declared absence.

Nicolas Maduro, who began his political career as a union leader for bus drivers, is currently minister of foreign affairs, and is the first person to have been named a successor in the 18 months since Chávez has been fighting cancer.

"If something were to happen that would make it impossible for me to be president, Nicolas Maduro should assume power for the short time left in this term. But also, it is my firm opinion, my complete and irrevocable opinion, that under this scenario, you should all vote for Nicolas", he said.

Surrounded by Maduro and several of his closest aides Chávez, who is scheduled to be sworn in for his fourth term 10 January, said he had been cancer-free at the time of inscribing his candidacy last June. But he said new malignant cells had been discovered following a thorough examination conducted last week in Havana, where the president flew to receive hyperbaric oxygen therapy.

"I insisted in having all of the medical checks before inscribing my candidacy and all of the results were positive. If there had been any negative results, be assured that I would have not inscribed my candidacy", he said.

This new set-back in the 58 year-old leader's health comes one week ahead of regional polls where Venezuelans will be electing governors and mayors.

During the broadcast, Chávez emphatically called on his supporters to remain together, warning them of attempts by the opposition to destroy the gains of his revolution.

Calling upon the sword of Simon Bolivar, the nation's independence hero, Chávez expressed his sorrow for having to convey to the nation the news of the new-found cancer.

"This situation pains me deeply because of the pain it causes to millions of you. But as I've said before, Chávez is no longer this human being, he is a collective", he said.